This invention relates to a bar screen with a screen cleaner for waste water treatment installations wherein a carriage is mounted for movement by a reversing drive in guides which extend parallel to the trash rack for the bar screen and a rake is pivotally supported by means of pivot arms on the carriage with the rake being moved toward the trash rack as it moves upward and pivoted out to a release position away from the trash rack as it moves downwardly. Such prior art trash rack cleaners have also included an articulated lever arranged between the carriage and the pivot arm which determines the position of the rake when either extended or articulated and which, when the carriage reaches the lowest point of its downward movement, contacts a lower permanent stop to control the motion of the rake.
A prior art type trash rack cleaner is also disclosed in German Pat. No. 1,252,150 wherein the articulated lever is so arranged that during downward motion it holds the rake in the release position, and immediately upon reaching the lower changeover point is articulated by a permanent tripping member so that the rake can be brought into the forward position at the trash rack. This forward movement of the rake occurs initially under its own weight. Also, the hoist device driving the carriage can be changed on the carriage and connected to the pivot arm of the rake so that as the rake is moved upward it is drawn towards the trash rack by means of an additional force corresponding to the weight of the carriage and the rake.
With similar prior art cleaning rakes which are pivoted by means of their own weight against the trash rack, the danger arises that the pivoting force of the rake is not sufficient to collect all of the debris on the water surface in front of the trash rack. In particular, it is possible that the rake would not penetrate the floating debris until it starts to move upwardly when the full weight of the carriage takes effect on the rake by the hoist device, so that uncleared remains of floating debris remain at the bottom of the trash rack.